Unable to switch across tabs (Desktop, Panel, Windows) by using only keyboard

[Norbert]

Steps to reproduce:
1. Have Ubuntu MATE 18.04 installed
2. Launch MATE Tweak -> the Desktop tab will be opened by default
3. Try to switch to other tabs by only using keyboard, i.e. with <Tab> key

Expected result:
* user is able to switch from Desktop tab to other tabs and move across tabs

Actual result:
* user is unable to switch tabs of MATE Tweak, user got stuck on Desktop tab

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: mate-tweak 18.04.16-1
ProcVersionSignature: Ubuntu 4.15.0-46.49-generic 4.15.18
Uname: Linux 4.15.0-46-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.6
Architecture: amd64
CurrentDesktop: MATE
Date: Fri Mar 29 23:19:26 2019
InstallationDate: Installed on 2018-04-26 (336 days ago)
InstallationMedia: Ubuntu-MATE 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
PackageArchitecture: all
SourcePackage: mate-tweak
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

[Expired for mate-tweak (Ubuntu) because there has been no activity for 60 days.]